The reinforcing structure or reinforcement of tires, particularly of tires for vehicles of the heavy vehicle type, is currently—and usually—made up of a stack of one or more plies conventionally known as “carcass plies”, “crown plies”, etc. This way of naming the reinforcements stems from the method of manufacture, which involves creating a series of semi-finished products in the form of plies, provided with thread-like reinforcements, often longitudinal, which are then assembled or stacked in order to build a green tire. The plies are produced flat, with substantial dimensions, and are then cut to the dimensions of a given product. The plies are also initially assembled substantially flat. The green tire thus produced is then shaped to adopt the toroidal profile typical of tires. The semi-finished “finishing” products are then applied to the green tire in order to obtain a product ready to be cured.
A “conventional” type of method such as this involves, particularly during the phase of manufacturing the green tire, the use of an anchoring element (generally a bead wire) which is used to anchor or retain the carcass reinforcement in the bead region of the tire. Thus, for this type of method, a portion of all the plies that make up the carcass reinforcement (or just some of them) is folded back around a bead wire positioned in the bead of the tire. That then anchors the carcass reinforcement in the bead.
The widespread use across industry of this conventional type of method, in spite of there being numerous variations in how the plies are created and assembled, has led those skilled in the art to adopt a vocabulary based on the method; hence the terminology generally used, involving in particular the terms “plies”, “carcass”, “bead wire”, “shaping” to denote the transition from a flat profile to a toroidal profile, etc.
Nowadays there are tires which do not strictly speaking have “plies” or “bead wires” as understood from the above definitions. For example, document EP 0 582 196 describes tires manufactured without the aid of semi-finished products in the form of plies. For example, the reinforcing elements of the various reinforcing structures are applied directly to the adjacent layers of rubber compounds, everything being applied in successive layers to a toroidal core, the shape of which directly yields a profile similar to the final profile of the tire that is in the process of being manufactured. Thus, in this case, there are no longer any “semi-finished” products or “plies”, or “bead wires”. The basic products such as the rubber compounds and the reinforcing elements in the form of threads or filaments are applied directly to the core. As this core is of a toroidal shape, there is no longer any need to form the green tire in order to change from a flat profile to a profile in the form of a torus.
Furthermore, the tires described in that document do not use the “traditional” turning back of the carcass ply around a bead wire. That type of anchorage is replaced by an arrangement in which circumferential threads are positioned adjacent to the said sidewall reinforcing structure, everything being embedded in an anchoring or bonding rubber compound.
There are also methods of assembly on a toroidal core that employ semi-finished products specially designed for rapid, effective and simple laying onto a central core. Finally, it is also possible to use a hybrid comprising both certain semi-finished products for achieving certain architectural aspects (such as plies, bead wires etc.), while others are created by applying compounds and/or reinforcing elements directly.
In this document, in order to take account of recent technological advances both in the field of manufacture and in the design of the products, the conventional terms such as “plies”, “bead wires”, etc., are advantageously replaced by terms which are neutral or independent of the type of method used. Hence, the term “carcass type reinforcement” or “sidewall reinforcement” can validly be used to denote the reinforcing elements of a carcass ply in the conventional method, and the corresponding reinforcing elements, generally applied to the sidewalls, of a tire produced according to a method that does not involve semi-finished products. The term “anchoring zone” for its part, can just as easily denote the “traditional” turning back of the carcass ply around a bead wire in a conventional method as it can the assembly formed by the circumferential reinforcing elements, the rubber compound and the adjacent sidewall reinforcing portions of a bottom region created using a method which involves applying elements to a toroidal core.
In general, in tires of the heavy vehicle tire type, the carcass reinforcement is anchored on each side in the region of the bead and is surmounted radially by a crown reinforcement, constituted by at least two layers that are superposed and formed of threads or cords which are parallel within each layer and crossed from one layer to the next making with the circumferential direction angles comprised between 10° and 45°. The said working layers, that form the working reinforcement, may also be covered by at least one so-called protective layer formed of reinforcing elements, which are advantageously metal and extensible, known as elastic reinforcing elements. It may also comprise a layer of metal cords or threads with low extensibility, that make an angle comprised between 45° and 90° with the circumferential direction, this ply, known as a bracing ply, being situated radially between the carcass reinforcement and the first so-called working crown ply, formed of parallel cords or threads at angles of at most 45° in terms of absolute value. The bracing ply forms, with at least the said working ply, a triangulated reinforcement which, under the various stresses that it experiences, undergoes very little deformation, the essential role of the bracing ply being to react transverse compressive forces to which all of the reinforcing elements are subjected in the region of the crown of the tire.
In the case of heavy vehicle tires, a single protective layer is usually present and its protective elements are, mostly, oriented in the same direction and at the same angle in terms of absolute value as those of the reinforcing elements of the radially outermost and therefore radially adjacent working layer. In the case of engineering works vehicle tires intended to run on somewhat uneven ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements of the radially interior protective layer being crossed with the inextensible reinforcing elements of the working layer that is radially exterior and adjacent to the said radially interior protective layer.
Cords are said to be inextensible when the said cords have, under a tensile force equal to 10% of the breaking strength, a relative elongation of 0.2% at most.
Cords are said to be elastic when the said cords have, under a tensile force equal to the breaking strength, a relative elongation at least equal to 3% with a maximum tangent modulus of less than 150 GPa.
Circumferential reinforcing elements are reinforcing elements which make, with the circumferential direction, angles comprised in the range +2.5°, −2.5° about 0°.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis about which it revolves under normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential median plane or equatorial plane is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
What is meant by the “elastic modulus” of a rubber compound is a secant tensile modulus at 10% deformation and ambient temperature.
As far as the rubber compounds are concerned, modulus measurements are carried out under tension in accordance with AFNOR-NFT-46002, September 1988: the nominal secant modulus (or apparent stress, in MPa) is measured in second elongation (i.e. after an accommodating cycle) at 10% elongation (standard temperature and hygrometry conditions in accordance with AFNOR-NFT-40101, December 1979).
As far as the metal threads or cords are concerned, the measurements of load at break (maximum load in N), strength at break (in MPa) and elongation at break (total elongation in %) are performed under tension in accordance with ISO 6892, 1984.
Some present-day tires, known as “road” tires, are intended to run at high speed and over increasingly long distances, because of the improvements to the road network and the expansion of the motorway network worldwide. Although all of the conditions under which a tire is called upon to run undoubtedly allows an increase in the number of kilometers covered, because tire wear is lower, this is at the expense of tire durability, particularly of crown reinforcement durability.
The issue is that there are stresses in the crown reinforcement and more particularly shear stresses between the crown layers, combined with a not insignificant increase in operating temperature at the ends of the axially shortest crown layer, which result in the appearance and propagation of cracks in the rubber at the said ends. The same problem exists in the case of edges of two layers of reinforcing elements, the said other layer not necessarily being radially adjacent to the first.
To improve the endurance of the crown reinforcement of the type of tire being studied, solutions relating to the structure and quality of the layers and/or profiles of rubber compounds which are positioned between and/or around the ends of plies and, more particularly, the ends of the axially shortest ply, have already been applied.
Patent FR 1 389 428, in order to increase the resistance to damage of the rubber compound situated near the edges of the crown reinforcement, recommends the use, in combination with a low-hysteresis tread, of a rubber profile covering at least the sides and the marginal edges of the crown reinforcement and consisting of a low-hysteresis rubber compound.
Patent FR 2 222 232, in order to avoid separation between crown reinforcement plies, teaches the coating of the ends of the reinforcement in a rubber mat, the Shore A hardness of which differs from that of the tread surmounting the said reinforcement, and is higher than the Shore A hardness of the profile of rubber compound positioned between the edges of crown reinforcing plies and carcass reinforcement.
French application FR 2 728 510 proposes positioning, on the one hand, between the carcass reinforcement and the crown reinforcement working ply radially closest to the axis of rotation, an axially continuous ply formed of inextensible metal cords that make an angle of at least 60° with the circumferential direction and the axial width of which is at least equal to the axial width of the shortest working crown ply and, on the other hand, between the two working crown plies, an additional ply formed of metal elements directed substantially parallel to the circumferential direction.
Prolonged running of the tires thus constructed under particularly harsh conditions has revealed limits in terms of the endurance of these tires.
To remedy such disadvantages and improve the endurance of the crown reinforcement of these tires, it has been proposed that there be associated with the angle working crown layers at least one additional layer of reinforcing elements substantially parallel to the circumferential direction. French application WO 99/24269 proposes, notably, on each side of the equatorial plane and in the immediate axial continuation of the additional ply of reinforcing elements substantially parallel to the circumferential direction, that the two working crown plies formed of reinforcing elements that are crossed from one ply to the next be coupled over a certain axial distance and then decoupled by profiles of rubber compound at least over the remainder of the width common to the said two working plies.
The layer of circumferential reinforcing elements usually consists of at least one metal cord wound to form a spiral laid at an angle of less than 8° with respect to the circumferential direction. The cords initially manufactured are coated with a rubber compound before being laid. This rubber compound then penetrates the cord under the effect of the pressure and the temperature when the tire is cured. The coating of the cords with the rubber compound may be performed in an intermediate phase between manufacturing the cord and laying it to be stored in the form of a reel. An alternative form of manufacture is to coat the cords with the rubber compound at the same time as or, more precisely, just before, the said cords are laid.
The results obtained in terms of endurance and wear during prolonged running on high-speed routes are mostly satisfactory. Nonetheless, it has emerged that under certain running conditions, some tires exhibit more pronounced wear on one part of their tread, in particular the centre. This phenomenon becomes more pronounced as the width of the tread increases.